A-c voltage detector

ABSTRACT

A circuit for indicating the presence or absence of an a-c voltage. A comparator assumes a first state when an a-c voltage applied thereto is outside a predetermined range of threshold voltages and assumes a second state when the a-c voltage is within that range of voltages. An output voltage control circuit establishes a first output voltage, to indicate the presence of an a-c input voltage, when the comparator is in its first state and establishes a second output voltage, to indicate the absence of an a-c voltage, when the comparator is in its second state. Circuitry is provided to introduce a time-delay between the time that the comparator assumes its second state and the time that the output voltage control circuit establishes the second value of output voltage. The latter time-delay is arranged to be short enough to allow the voltage detector to rapidly indicate actual interruptions in the a-c input voltage but long enough to prevent the voltage detector from indicating an a-c voltage interruption merely because the a-c voltage has varied through the above voltage range in the course of normal a-c voltage variations.

United States Patent 1 Mesenhimer 1 Apr. 3, 1973 [54] A-C VOLTAGEDETECTOR [75] Inventor: Lee 0. Mesenhimer, Avon, Ohio [58] Field ofSearch ..307/230, 235; 328/147, 149, 328/150, 135

[56] References Cited UNITED STATES PATENTS 3,573,638 4/1971 Cox, Jr. etal. ..307/235 X 3,482,116 12/1969 James ..307/235 X 3,626,214 12/1971Wesner ..307/235 R OTHER PUBLICATIONS R. .l. Widlar et al., Designingwith Off-the-Shelf Linear Microcircuits, Application Bulletin APP-124,Fairchild Semiconductor, pg.7, January, 1966.

Primary Examiner.lohn Zazworsky AttorneyJohn Howard Smith [57] ABSTRACTA circuit for indicating the presence or absence of an a-c voltage. Acomparator assumes a first state when an a-c voltage applied thereto isoutside a predetermined range of threshold voltages and assumes a secondstate when the a-c voltage is within that range of voltages. An outputvoltage control circuit establishes a first output voltage, to indicatethe presence of an a-c input voltage, when the comparator is in itsfirst state and establishes a second output voltage, to indicate theabsence of an a-c voltage, when the comparator is in its second state.Circuitry is provided to introduce a time-delay between the time thatthe comparator assumes its second state and the time that the outputvoltage control circuit establishes the second value of output voltage.The latter time-delay is arranged to be short enough to allow thevoltage detector to rapidly indicate actual interruptions in the a- 0input voltage but long enough to prevent the voltage detector fromindicating an a-c voltage interruption merely because the a-c voltagehas varied through the above voltage range in the course of normal a-cvoltage variations.

12 Claims, 4 Drawing Figures PATENTEUAPR3 1975 saw 1 or 2 ATTORNEYPATENTEDAPR3 I973 3,725,795

SHEET 2 OF 2 INVENTOR:

LEE 0. MESENHIMER ATTORNEY A-C VOLTAGE DETECTOR BACKGROUND OF THEINVENTION The present invention relates to detector circuits and isdirected more particularly to circuitry for detecting the presence orabsence of an a-c voltage.

Under circumstances where the presence or absence of an a-c voltage isindicative of the operative condition of an electrical circuit, it isoften necessary to provide a-c voltage detection circuitry which cangive a rapid indication of an interruption or restoration of that a-cvoltage. An indication that an a-c voltage has been interrupted may, forexample, be utilized to activate circuitry for disconnecting aninoperative source and for substituting therefor an operativereplacement. An a-c voltage detection circuit may also be used to turnon ac voltage failure alarms.

Prior to the present invention, a-c detection circuits have presentedseveral problems. One problem was that they respond relatively slowly tochanges in the a-c voltage applied thereto. This slow response causedthe desired indication of a change in the condition of the ac voltage tobe delayed for from one to several a-c voltage cycles. As a result, suchcircuitry was unsuitable for use in connection with many presentlyavailable highspeed monitoring and control circuits.

Another problem was that many a-c detector circuits required the flow ofappreciable a-c current from the ac circuit being monitored. Thisprevented the a-c detection circuit from being utilized to monitor a-csignals of low power content as, for example, certain audio frequencysignals.

Still another problem was that other, more sensitive a-c voltagedetector circuits were unable to distinguish the a-c signal beingmonitored from undesired low level signals such as induced 6O cycle hum.As a result, indications of the presence or absence of the desired a-csignal were often indistinguishable from indications of the presence orabsence of unwanted signal noise.

With the present invention there is afforded a-c voltage detectioncircuitry which can indicate changes in the condition of an a-c voltagesource within a fraction of one-half cycle of the voltage thereof, whichdraws negligible a-c current from the source being monitored and whichis immune to low level signal noise.

SUMMARY OF THE INVENTION It is an object of the invention to provide animproved a-c voltage detector.

Another object of the invention is to provide an a-c voltage detectorwhich can indicate the termination or appearance of an a-c voltageduring either half-cycle thereof.

Yet another object of the invention is to provide an a-c voltagedetector which is insensitive to unwanted low level signals, such as,for example, signals which appear in the circuit being monitored as aresult of coupling between that circuit and circuits not beingmonitored. This is accomplished by establishing (about the zero value ofthe a-c input voltage) a threshold voltage range and by preventing thecircuitry from indicating the appearance of an a-c input voltage so longas the instantaneous value of the a-c input voltage is within thatthreshold voltage range.

Still another object of the invention is to provide an a-c voltagedetector which delays the indication of an interruption in an a-cvoltage for a time sufficient for the a-c voltage being monitored tovary between the limits of the above threshold voltage range. This delayallows the voltage being monitored to pass through zero, during thecourse of ordinary a-c polarity reversals, without causing the detectorto indicate an interruption of that voltage.

It is another object of the invention to provide an a-c voltage detectorwhich indicates the appearance of an a c voltage without regard to thetime-delay associated with the indication of an interruption in the a-cvoltage.

A further object of the invention is to provide an a-c voltagedetectorwhich draws negligible current from the a-c source beingmonitored, that is, which has a high input impedance.

A still further object of the invention is to provide a voltage detectorcircuit wherein one a-c input terminal and one output terminal may beconnected to a common ground.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of one circuitembodying the invention,

FIGS. 2a and 2b are graphs showing various voltages appearing in thecircuit of FIG. 1 as a function of time, and

FIG. 3 is a schematic diagram of another circuit embodying theinvention.

DESCRIPTION OF THE INVENTION Referring to FIG. 1, there is shown an a-cvoltage detector circuit 10 for monitoring the voltage of an a-c source1 l. The latter source may include an a-c generator, an inverter or, ingeneral, any network which can establish an a-c voltage at inputterminals 12 and 13 of detector 10. If source 11 provides a mixed a-cand d-c voltage, the a-c component of the latter may be detectedseparately from the d-c component thereof by connecting a d-c blockingcapacitor 14 between source 1 1 and detector circuit 10.

To the end that the circuit of FIG. 1 may detect the presence or absenceof an a-c voltage during either half-cycle thereof, there is providedcomparing means including a positive voltage comparator 15 which isresponsive to positive half-cycles of the a-c input voltage and anegative voltage comparator 16 which is responsive to negativehalf-cycles of the a-c input voltage. Comparators 15 and 16 may compriseoperational amplifiers of suitably high input impedance havingrespective non-inverting inputs 15a and 16a, respective inverting inputs15b and 16b and respective outputs 15c and 160. In the embodiment shownin FIG. 1, amplifiers 15 and 16 consist of integrated circuits of thetype which generate only a positive-going output voltage in the absenceof an external pull-down resistor such as resistor 34. It will beunderstood, however, that amplifiers l5 and 16 may also compriseoperational amplifiers which generate both positive and negativegoingoutput voltages if the negative-going output voltage are blocked bysuitable diodes.

An important advantage of the present invention is that the common ofthe circuit can be at the same potential and connected to the ground ofthe a-c source being monitored. This presence the output voltage fromfloating at random with respect to the a-c source ground. The manner ofattaining this advantage will now be described.

Operative power is supplied to amplifiers l5 and 16 from suitable d-cvoltage supplies 18a and 18b through conductor pairs 20 and 21 and 22and 23. The positive lead of source 18b is connected to the negativelead of source 18a at a junction C which has been selected for use as acircuit common. It will be understood that all points which are labeledC are at the potential of the circuit common. Thus, the ground of a-csource 11 is at the same potential as detector circuit output terminal36.

One problem with sensitive a-c voltage detectors is that they tend togive erroneous information in the presence of undesired low levelvoltages such as noise or induced 60 cycle hum. In order that thecircuit of the invention may be insensitive to such low level voltagesand yet may be sensitive to the voltage which is to be detected, thereare introduced input voltage thresholds whereby undesired low levelvoltages are prevented from affecting the operation of the detector. Inthe present embodiment, these input voltage thresholds are establishedby applying predetermined d-c'threshold voltages to comparator inputs band 16a, this being accomplished by connecting the latter inputs to d-cvoltage dividers including resistor pairs 27 and 28 and 29 and 30 whichserve as threshold voltage supply means. The a-c voltage which must beapplied to amplifier inputs 15a and 16b to overcome these d-c thresholdsmay be preset by connecting an a-c voltage divider, including resistors25 and 26, between input terminals 12 and 13 and comparator inputs 15aand 16b through signal input conductors 32, 33a and 33b.

If, for example, the voltage which source 11 establishes across resistor26, that is, between common C and amplifier input 15a, exceeds thepositive threshold voltage which voltage divider 27-28 establishesbetween common C and amplifier input 15b, amplifier output 150 will riseto a positive value nearly equal the potential of positive supplyconductor 20. This occurs because the relatively high gain of amplifier15 causes even a small positive voltage between input 15b and input 15ato drive amplifier output 150 to the positive limit of its outputvoltage.

If, on the other hand, the positive voltage which source 11 establishesacross resistor 26 is less than the positive threshold voltage atamplifier input 15b, the potential of amplifier output 15: will fallbelow that of common C. (It is assumed for the sake of description thata capacitor 42, to be described later, has no effect at this time). Thisoccurs because, as previously described, amplifier 15 can produce only apositive output voltage and because, in the absence of that positivevoltage, output 150 is pulled-down toward the potential of conductor 22by pull-down resistor 34. Thus, amplifier l5 acts as a differentialswitching circuit in that it switches output terminal 150 between apositive and negative potential with respect to common C in accordancewith the sign of the difference between the voltages at inputs 15a and15b.

Similarly, the potential of amplifier output 160 is either positive ornegative with respect to common C depending upon whether the negativevoltage which source 11 establishes at amplifier input 16b is greater orless than the negative threshold voltage which voltage divider 29-30establishes at amplifier input 16a.

From the foregoing, it will be seen that a junction J 1 which isconnected to both amplifier outputs will be negative from common C whensource 11 causes the ac voltage of signal input conductor 32 to be lessthan the threshold voltage at amplifier input 15b as well as less thanthe threshold voltage at amplifier input 16a, and that junction J willbe positive from common C when the a-c voltage of signal input conductor32 exceeds either the positive threshold voltage at input 15b or thenegative threshold voltage at input 160. In other words, junction J willbe negative when the 21-0 voltage of signal input conductor 32 is withinthe range of voltages between the above positive and negative thresholdvoltages and will be positive when the a-c voltage on conductor 32 isnot within that range of voltages. Accordingly, if the positive andnegative threshold voltages are preset at values which are less than thepeak positive values of the a-c values of the a-c voltage on conductor32 but which are greater than the peak values of any undesired low-levelvoltages, junction J will become'positive from common C only when thea-c voltage being monitored is present between a-c input terminals 12and 13. Thus, a positive voltage at junction J indicates that thedesired a-c voltage is present.

The presence of a negative voltage at junction I, does not, however,necessarily indicate that an a-c voltage is not present. This is becausea negative voltage at junction J 1 may, on the one hand, indicate thatno a-c voltage is present between terminals 12 and l3 and may, on theother hand, indicate that an a-c voltage is present but that itsinstantaneous value is such that the voltage of conductor 32 istemporarily between the above positive and negative threshold voltagesas, for example, during the time just before and just after the a-cinput voltage reverses polarity. To the end that these two negativevoltage conditions may be distinguished, there is provided discriminatorcircuitry which establishes a first predetermined voltage betweendetector output terminals 35 and 36 when junction J l is positive fromcommon C and which establishes a second predetermined voltagetherebetween when junction'J becomes negative from common C for a timesufficient to indicate that there has occurred more than just a reversalin the polarity of the a-c input voltage. In other words, detector 10will not establish its second predetermined output voltage, to indicatean interruption in the a-c input voltage, merely because there hasoccurred a reversal in the polarity of the a-c input voltage. Thus,actual as opposed to apparent a-c vvoltage interruptions aredistinguished.

In the present embodiment, the discriminator cir cuitry includes voltagecontrol means, comprising an NPN transistor 38 and a resistor 39, forcontrolling the voltage between output terminals 35 and 36 in accordancewith the voltage at junction J that is, in accordance with whether ornot the a-c voltage on signal input conductor 32 is or is not within thethreshold voltage range set by voltage dividers 27-28 and 29-30. Theabove circuitry also includes time-delay means, comprising a capacitor42 and resistors 34 and 40 for modifying the control activity oftransistor 38 to afford the desired time-delay between the time that thea-c input signal assumes a value within the above threshold voltagerange and the time that there occurs an indication of an a-c voltageinterruption at terminals 35 and 36.

The operation of the above circuitry will now be described. Referring toFIG. 2a, there is shown V the voltage on input conductor 32 as afunction time. Also shown are the positive and negative thresholdvoltages, V and V which appear at amplifier inputs 15b and 16a,respectively. Before time T,, when no a-c voltage appears on inputconductor 32, the a-c voltage applied to neither amplifier exceeds thethreshold voltage applied to the same amplifier, with the result thatjunction 1 is negative from common C. Under these conditions, currentflows from d-c source 18b through the path including conductor 45, adiode 47, a conductor 43, resistor 34 and conductor 22. This currentrenders junction J negative from common C by a voltage equal to theforward voltage drop across diode 47. Since junction J is connected tothe base of transistor 38 and since common C is connected to the emitterthereof,

NPN transistor 38 is non-conducting. This, in turn, causes outputterminal 35 to be at the potential of positive supply conductor 20 andthereby indicate the absence of an -a-c voltage between input terminals12 and 13. The potential of output tenninal 35, V is illustrated in FIG.2b.

Between times T and T when the voltage on input conductor 32 becomespositive but remains between the positive and negative threshold voltageat amplifier inputs 15b and 16a, junction J remains negative from commonC. As a result, output terminal 35 remains at the potential of conductor20 to continue the indication that no a-c voltage is present between a-cinput terminals 12 and 13. This merely reflects the effect of thethreshold circuitry which allows the rejection of extraneous low levelsignals.

After time T when the voltage on input conductor 32 rises above thethreshold voltage at amplifier input 15b, amplifier output 150 andjunction J rise toward the voltage of positive supply conductor 20, aspreviously described. Because capacitor 42 is connected as shown in FIG.1, however, there is a time-delay between the appearanceof apositive-going voltage at junction J and the resultant conduction ofoutput voltage control transistor 38. This time-delay causes transistor38 to approach conduction over a period of 45 time as capacitor 42charges to a voltage which renders junction J 1 sufficiently positive'from common C to reverse bias diode 47 and forward bias transistor 38.The length of this time-delay is, of course, determined by thecapacitance of capacitor 42, the initial charge thereon and the rate atwhich charging current can be supplied from amplifier 15 throughconductor 43. It will be understood that if amplifier 15 (or 16) doesnot have internal current limiting circuitry, it may be necessary toconnect a resistor in series with conductor 43 to limit the chargingcurrent which capacitor 42 draws from that amplifier.

In accordance with one feature of the present invention, the abovetime-delay is caused to occupy a relatively small fraction of onehalf-cycle of the a-c voltage by pre-conditioning the time-delaycircuitry to allow a relatively rapid change in the conduction oftransistor 38 upon the appearance of an a-c input voltage. In thepresent embodiment, this is accomplished by precharging capacitor 42 toa voltage such that only a relatively small additional charge isnecessary to bring transistor 38 to the threshold of conduction. Thisprecharging is provided by a clamping diode 47 which acts in conjunctionwith source 18b to prevent the potential of the upper plate of capacitor42 (and junction J from dropping below the potential of common C by morethan a fixed voltage, approximately six-tenths of 1 volt in the presentembodiment. As a result, capacitor 42 need only be charged to anadditional 1.2 volts before it renders junction J sufficiently positivefrom common C to forward bias transistor 38 and thereby initiate theactivity thereof which culminates in the indication of the appearance ofan a-c input voltage. An exemplary value of the time required forcapacitor 42 to charge to the voltage necessary to forward biastransistor 38 is shown by the time interval T -T of FIG. 2b. The lattertime interval may, of course, be lengthened by omitting clamping diode47 or by introducing a suitable charging current limiting resistor inseries with conductor 43.

After time T as capacitor 42 charges to a still higher voltage throughjunction J the conduction of transistor 38 increases, thus causing thevoltage between output terminals 35 and 36 to approach zero.

Transistor 38 attains full conduction at time T at which time thevoltage between output terminals 35 and 36 is approximately zero,indicating that an a-c voltage is present. Thereafter, as capacitor 42con-- tinues to charge, the voltage at junction J attains its maximumvalue with respect to common C. Thus, soon after time T transistor 38attains full conduction and capacitor 42 overcharges to raise thepotential of junction J, to a value substantially in excess of thatrequired to maintain full conduction through transistor 38.

Referring to FIGS. 2a and 2b, it will be seen that the above describeddetection of the appearance of an a-c voltage occurs within one-quarterof one half-cycle of the a-c input voltage wave. It will be understoodthat this time may be shortened or lengthened, depending upon the valuesselected for the circuit elements of FIG. 1.

The above conditions will continue until time T, when the voltage oninput conductor 32 drops below the positive threshold voltage atamplifier input 15b. Between that time and time T,, when the voltage oninput conductor 32 rises above negative threshold voltage at amplifierinput 16a, neither amplifier 15 nor amplifier 16 can maintain thevoltage at junction J l at its previously assumed positive value. As aresult, during time interval T -T the circuit of FIG. 1 begins to returnto the conditions which were present before time T In accordance withone feature of the present invention, the latter return is delayed for atime greater than the time required for the a-c voltage on conductor 32to vary from a value equal to the positive reference voltage to a valueequal to the negative reference voltage. This allows the circuit of FIG.1 to withhold an indication of an a-c voltage interruption when the a-cvoltage is undergoing a polarity reversal and yet allows the circuit ofFIG. 1 to indicate actual a-c voltage interruptions soon after theyoccur.

In the present embodiment, the desired time-delay introduced to preventfalse indications during a-c input voltage polarity reversals isaccomplished by the introduction of capacitor 42 and resistors 40 and34, as will now be described. Between times T and T neither amplifier 15nor amplifier 16 can maintain a positive voltage at junction J As aresult, the voltage at the latter junction begins to decrease toward thevalue at which transistor 38 will turn off and thereby indicate aninterruption in the a-c input voltage. The time required for thisdecrease to occur is extended, for a time, by capacitor 42 whichsupports the voltage at junction J 1 by discharging through a first pathincluding resistor 40, the base-emitter circuit of transistor 38 andsource 19 and through a second path including conductor 43 and resistor34. Because capacitor 42 is, at this time, overcharged, that is, chargedto a voltage substantially in excess of that required to maintainjunction J l at a voltage sufficient to support full conduction throughtransistor 38, transistor 38 will remain fully conductive during timeinterval T,,T As a result, the voltage between output terminals 35 and36 is prevented from even beginning to rise to the potential of positivesupply conductor 20 during an a-c reversal. Thus, the circuit of FIG. 1does not indicate an a-c voltage interruption during what is merely ana-c voltage reversal.

After an actual interruption in the a-c input voltage as, for example,the interruption which begins at time T in FIGS. 2a and 2b, transistor38 begins to turn off in the manner described in connectionwith timeinterval T -T6.,. Because the interruption does not terminate within aninterval such as T5--T6, however, the timedelay provided by capacitor 42soon exhausts itself. This results in the turn-off of transistor 38 andthe resultant indication of an interruption of the a-c input voltage. Inthe example shown in FIG. 2b, the timedelay between an a-c voltageinterruption and the resultant indication thereof at terminals 35 and 36spans the time interval T T the latter having a duration approximatelyequal to one-quarter of one cycle of the input voltage. The duration ofthe turn-off time interval may, of course, be made shorter or longer byselecting different values for the circuit elements of FIG. 1.

The rise in output voltage which follows the a-c voltage interruption attime T occurs in two stages, the first stage occurring between times Tand t and the second stage occurring between times T and T During thefirst stage, capacitor 42 is discharging but is able to maintain atjunction J 1 a voltage which will support full conduction throughtransistor 38. During the second stage, capacitor 42 has discharged tothe point where continued discharge causes transistor 38 to becomeincreasingly non-conductive, thus causing the voltage between terminals35 and 36 to approach the voltage of positive supply conductor 20. Aftertransistor 38 turns fully off, capacitor 42 continues to discharge untilthe potential of the upper plate thereof falls to a value (in thepresent embodiment) of approximately 0.6 volts negative from common C,the latter being the factor which initiates the previously describedclamping activity of diode 47. When this occurs, the circuit of FIG. 1is in the condition which it was in prior to time T and is ready todetect the reappearance of an a-c input voltage.

If it is desirable to utilize circuitry having only one voltagecomparator and one two-terminal d-c voltage supply, this may beaccomplished by arranging the circuitry of the invention in the mannershown in FIG. 3. The latter circuit is in many respects similar to thecircuit of FIG. 1 and like functioning parts are similarly numbered.

The circuit of FIG. 3 differs from that of FIG. 1 principally in thatthe comparing means of FIG. 3 comprises an operational amplifier 17 theoutput of which can vary in either direction from its quiescentpotential and the inputs 17a and 17b of which serve alternately andseverally as threshold inputs and signal inputs. When, for example, a-csource 11 drives input terminal 12 positive from input terminal 13,amplifier input 17a serves as a signal input due to the conduction of adiode 50a while amplifier input 17b serves as a threshold input due tothe non-conduction of a diode 50b and the d-c voltage divider action ofresistors 27 and 28. When, however, source 11 drives terminal 12negative from terminal 13, amplifier input serves as a signal input dueto the conduction of diode 50b while amplifier input 170 serves as athreshold input due to the non-conduction of diode 50a and the d-cvoltage divider action of resistor 30' and 29'.

To the end that the potential of junction J 1 of FIG. 3 may vary withthe a-c input voltage in the manner described previously in connectionwith junction J of FIG. 1, the voltage which voltage divider 27'28'establishes at input 17b when no a-c input voltage is present (i.e. thequiescent voltage at input 17b) is set at a value greater than thequiescent voltage which voltage divider 3029 establishes at input 17a.In addition, the quiescent voltage of a junction J, is set, by a voltagedivider comprising resistors 51 and 52, at a voltage between thequiescent voltages appearing at amplifier inputs 17b and 17a. Thus, whenthe a-c input voltage has an instantaneous value of zero, the potentialof a junction J, is between that of amplifier inputs 17a and 17b,voltage divider 5l52 serving to introduce into the a-c input voltage thed-c level necessary to produce this condition.

The above voltage relationships cause amplifier out-' put 17c andjunction J 1 to assume a potential near that of positive supplyconductor 20 when the voltage of junction J is either greater than thequiescent voltage of amplifier input 17a or less than the quiescentvoltage of amplifier input 17b. These relationships also cause amplifieroutput 170 and junction J to assume a potential near that of ground Gwhen the voltage at junction J: is between the quiescent thresholdvoltages at amplifier inputs 17a and 17b. Thus, the potential ofjunction J of FIG. 3 tends to assume one or the other of two valuesdepending upon whether the a-c input voltage is or is not within apreset range of threshold voltages.

The above described variations in the voltage at amplifier output 17cand junction J in turn, control the voltage which transistor 38establishes between output terminals 35 and 36. This control activity isaffected by time-delay capacitor 42 which operates in the mannerdescribed previously in connection with FIG. 1 to prevent the voltagebetween output terminals 35 and 36 from indicating interruptions in thea-c input voltage during a-c input voltage polarity reversals. Thus, thecircuit of FIG. 3 exhibits an operating characteristic substantially thesame as that previously in connection with FIGS. 1, 2a and 2b.

In view of the foregoing, it will be seen that an a-c voltage detectorconstructed in accordance with the invention is adapted to indicatechanges in the condition of an a-c input voltage within a fraction ofone halfcycle thereof and yet is adapted to avoid responding to thesignal noise or spikes which may accompany the a-c voltage beingmonitored. It will further be seen that the above described voltagedetecting activity occurs without regard to the polarity of the a-cinput at the time when the condition of the latter changes.

It will be understood that the embodiment shown herein is forillustrative purposes only and may be changed or modified withoutdeparting from the spirit and scope of the appended claims.

What is claimed is:

1. In a circuit for detecting the presence or absence of an a-c voltage,in combination, input terminal means, output terminal means, comparingmeans, said comparing means including signal input means, thresholdinput means and output means, threshold voltage supply means, means forconnecting said threshold voltage supply means to said threshold inputmeans to establish a threshold voltage range for said comparing means,means for connecting said input terminal means to said signal inputmeans, output voltage control means for establishing a first voltage atsaid output terminal means when the voltage at said signal input meansis outside said threshold voltage range and for establishing a secondvoltage at said output terminal means when the voltage at said signalinput means is within said threshold voltage range, means for connectingsaid voltage control means to the output means of said comparing meansand time-delay means for delaying the establishment of said secondvoltage for a time greater than the time required for the voltage atsaid signal input means to vary through said threshold voltage range.

2. A circuit as set forth in claim 1 in which said comparing meanscomprises an operational amplifier having non-inverting and invertinginputs which serve alternately and severally as signal input means andthreshold input means.

3. A circuit as set forth in claim 2 in which said means for connectingsaid input terminal means to said signal input means includesunidirectional conducting means.

4. A circuit as set forth in claim 1 in which said comparing meanscomprises first and second operational amplifiers each havingnon-inverting and inverting inputs, one input of each amplifier servingas threshold input means and the other input of each amplifier servingas signal input means.

5. A circuit as set forth in claim 1 in which one of said input terminalmeans is at the same potential as one of said output terminal means.

6. In a circuit for detecting the presence or absence of an a-c voltage,in combination, input terminal means, output terminal means, comparingmeans having signal input means, threshold input means and output means,threshold voltage supply means, means for connecting said thresholdvoltage supply means to said threshold input means to establish athreshold voltage range for said comparing means, means for connectingsaid input terminal means to said signal input means, said comparingmeans serving to establish a first voltage at the output means thereofwhen the voltage at said signal input means is outside said thresholdvoltage range and to establish a second voltage at the output meansthereof when the voltage at said signal input means is within saidthreshold voltage range, discriminator means for establishing a firstoutput voltage at said output terminal means when said first voltageappears at the output means of said comparing means and for establishinga second output voltage at said output terminal means when said secondvoltage appears at the output means of said comparing means for a timesufficient to allow an a-c input voltage polarity reversal to occur, andmeans for connecting said discriminator means to said comparing meansand to said output terminal means.

7. In a circuit for detecting the presence or absence of an a-c voltage,in combination, input terminal means, output terminal means, first andsecond differential switching means each having signal input means,threshold input means and output means, means for establishing thresholdvoltages at said threshold input means, means for connecting said inputterminal means to said signal input means, output voltage control meansfor establishing a first voltage at said output terminal means when thevoltage at either of said signal input means exceeds the voltage at therespective threshold input means and for establishing a second voltageat said output terminal means when the voltages at both of said signalinput means are less than the voltages at the respective threshold inputmeans, means for connecting said voltage control means to the outputmeans of said differential switching means and time-delay means fordelaying the establishment of said second voltage for a time greaterthan the time required for the voltage at said signal input means tovary between said threshold voltages.

8. A circuit as set forth in claim 7 including means for preconditioningsaid time-delay means to prevent the latter from appreciably affectingthe time required for said output voltage control means to establishsaid first voltage.

9. In a circuit for detecting the presence or absence of an a-c voltage,in combination, input terminal means, output terminal means, a positivecomparator and a negative comparator each having signal input means,threshold input means and output means, means for establishing positiveand negative threshold voltages at the threshold input means of saidpositive and negative comparators respectively, means for connectingsaid input terminal means to the signal input means of said comparators,output voltage control means for establishing a first voltage at saidoutput terminal means when the voltage at said signal input means isoutside the voltage range bounded by said positive and negativethreshold voltages and for establishing a second voltage at said outputterminal means when the voltage at said signal input means is withinsaid voltage range, time-delay means for delaying the establishment ofsaid second voltage for a time sufficient to allow the voltage at saidsignal input means to vary between said positive and negative thresholdvoltages during a-c polarity reversals and means for connecting saidtime-delay means between said voltage control means and the output meansof said comparators.

10. In a circuit for detecting the presence or absence of an a-cvoltage, in combination, input terminal means, output terminal means,first and second amplifying means each having signal input means,threshold input means and output means, means for establishing positiveand negative threshold voltages at the threshold input means of saidfirst and second amplifying means respectively, means for connectingsaid input terminal means to said signal input means, means forconnecting together the output means of said amplifying means, voltagesupply means having a positive terminal, a negative terminal and acommon electrically therebetween, means for connecting said positive andnegative terminals to said amplifying means, output voltage controlmeans for controlling the voltage between said output terminal means,means for connecting said voltage control means between said common andone terminal of said voltage supply means, capacitance means and meansfor connecting said capacitance means to said voltage control means andto the output means of said amplifying means.

l 1. A circuit for detecting the presence or absence of an a-c voltage,in combination, a-c input terminal means, output terminal means,differential switching means having first and second input means andoutput means, means for establishing a first quiescent threshold voltageat said first input means, means for establishing a second quiescentthreshold voltage at said second input means, means for connecting saida-c input terminal means to said first and second input means to applythereto respective positive and negative half-cycles of the a-c inputvoltage, means for introducing a predetermined d-c level into thevoltage applied to said first and second input means, output voltagecontrol means for establishing a first output voltage at said outputterminal means when the voltage applied to the input means of saiddifferential switching means is not between said first and secondquiescent threshold voltages and for establishing a second outputvoltage at said output terminal means when the voltage applied to theinput means of said differential switching means is between said firstand second quiescent threshold voltages, means for connecting saidoutput voltage control means the output means of said differentialswitching means and time-delay means for delaying the establishment ofsaid second output voltage for a time greater than the time required forthe voltage applied to the input means of said differential switchingmeans to vary between said first and second quiescent thresholdvoltages.

12. A circuit for detecting the presence or absence of an a-c voltage,in combination, a-c input terminal means, output terminal means,amplifying means having first and second input means and output means,means for establishing first and second quiescent threshold voltages atsaid first and second input means respectively, first unidirectionalconducting means for connecting said first input means to said a-c inputterminal means when the a-c voltage at said a-c input terminal means hasa first polarity, second unidirectional conducting means for connectingsaid second input means to said a-c input terminal means when thevoltage at said a-c input terminal means has a second polarity, d-cvoltage supply means, a voltage divider, means for connecting said inputterminal means to said d-c voltage supply means through said voltagedivider, output voltage control means, means for connecting said outputvoltage control means to said d-c voltage supply means an to the outputmeans of said ampli ying means, time-delay means and means forconnecting said time-delay means to the output means of the saidamplifying means and to said output voltage control means.

1. In a circuit for detecting the presence or absence of an a-c voltage,in combination, input terminal means, output terminal means, comparingmeans, said comparing means including signal input means, thresholdinput means and output means, threshold voltage supply means, means forconnecting said threshold voltage supply means to said threshold inputmeans to establish a threshold voltage range for said comparing means,means for connecting said input terminal means to said signal inputmeans, output voltage control means for establishing a first voltage atsaid output terminal means when the voltage at said signal input meansis outside said threshold voltage range and for establishing a secondvoltage at said output terminal means when the voltage at said signalinput means is within said threshold voltage range, means for connectingsaid voltage control means to the output means of said comparing meansand time-delay means for delaying the establishment of said secondvoltage for a time greater than the time required for the voltage atsaid signal input means to vary through said threshold voltage range. 2.A circuit as set forth in claim 1 in which said comparing meanscomprises an operational amplifier having non-inverting and invertinginputs which serve alternately and severally as signal input means andthreshold input means.
 3. A circuit as set forth in claim 2 in whichsaid means for connecting said input terminal means to said signal inputmeans includes unidirectional conducting means.
 4. A circuit as setforth in claim 1 in which said comparing means comprises first andsecond operational amplifiers each having non-inverting and invertinginputs, one input of each amplifier serving as threshold input means andthe other input of each amplifier serving as signal input means.
 5. Acircuit as set forth in claim 1 in which one of said input terminalmeans is at the same potential as one of said output terminal means. 6.In a circuit for detecting the presence or absence of an a-c voltage, incombination, input terminal means, output terminal means, comparingmeans having signal input means, threshold input means and output means,threshold voltage supply means, means for connecting said thresholdvoltage supply means to said threshold input means to establish athreshold voltage range for said comparing means, means for connectingsaid input terminal means to said signal input means, said comparingmeans serving to establish a first voltage at the output means thereofwhen the voltage at said signal input means is outside said thresholdvoltage range and to establish a second voltage at the output meansthereof when the voltage at said signal input means is within saidthreshold voltage range, discriminator means for establishing a firstoutput voltage at said output terminal means when said first voltageappears at the output means of said comparing means and for establishinga second output voltage at said output terminal means when said secondvoltage appears at the output means of said comparing means for a timesufficient to allow an a-c input voltage polarity reversal to occur, andmeans for connecting said discriminator means to said comparing meansand to said output terminal means.
 7. In a circuit for detecting thepresence or absence of an a-c voltage, in combination, input terminalmeans, output terminal means, first and second differential switchingmeans each having signal input means, threshold input means and outputmeans, means for establishing threshold voltages at said threshold inputmeans, means for connecting said input terminal means to said signalinput means, output voltage control means for establishing a firstvoltage at said output terminal means when the voltage at either of saidsignal input means exceeds the voltage at the respective threshold inputmeans and for establishing a second voltage at said output terminalmeans when the voltages at both of said signal input means are less thanthe voltages at the respective threshold input means, means forconnecting said voltage control means to the output means of saiddifferential switching means and time-delay means for delaying theestablishment of said second voltage for a time greater than the timerequired for the voltage at said signal input means to vary between saidthreshold voltages.
 8. A circuit as set forth in claim 7 including meansfor preconditioning said time-delay means to prevent the latter fromappreciably affecting the time required for said output voltage controlmeans to establish said first voltage.
 9. In a circuit for detecting thepresence or absence of an a-c voltage, in combination, input terminalmeans, output terminal means, a positive comparator and a negativecomparator each having signal input means, threshold input means andoutput means, means for establishing positive and negative thresholdvoltages at the threshold input means of said positive and negativecomparators respectively, means for connecting said input terminal meansto the signal input means of said comparators, output voltage controlmeans for establishing a first voltage at said output terminal meanswhen the voltage at said signal input means is outside the voltage rangebounded by said positive and negative threshold voltages and forestablishing a second voltage at said output terminal means when thevoltage at said signal input means is within said voltage range,Time-delay means for delaying the establishment of said second voltagefor a time sufficient to allow the voltage at said signal input means tovary between said positive and negative threshold voltages during a-cpolarity reversals and means for connecting said time-delay meansbetween said voltage control means and the output means of saidcomparators.
 10. In a circuit for detecting the presence or absence ofan a-c voltage, in combination, input terminal means, output terminalmeans, first and second amplifying means each having signal input means,threshold input means and output means, means for establishing positiveand negative threshold voltages at the threshold input means of saidfirst and second amplifying means respectively, means for connectingsaid input terminal means to said signal input means, means forconnecting together the output means of said amplifying means, voltagesupply means having a positive terminal, a negative terminal and acommon electrically therebetween, means for connecting said positive andnegative terminals to said amplifying means, output voltage controlmeans for controlling the voltage between said output terminal means,means for connecting said voltage control means between said common andone terminal of said voltage supply means, capacitance means and meansfor connecting said capacitance means to said voltage control means andto the output means of said amplifying means.
 11. A circuit fordetecting the presence or absence of an a-c voltage, in combination, a-cinput terminal means, output terminal means, differential switchingmeans having first and second input means and output means, means forestablishing a first quiescent threshold voltage at said first inputmeans, means for establishing a second quiescent threshold voltage atsaid second input means, means for connecting said a-c input terminalmeans to said first and second input means to apply thereto respectivepositive and negative half-cycles of the a-c input voltage, means forintroducing a predetermined d-c level into the voltage applied to saidfirst and second input means, output voltage control means forestablishing a first output voltage at said output terminal means whenthe voltage applied to the input means of said differential switchingmeans is not between said first and second quiescent threshold voltagesand for establishing a second output voltage at said output terminalmeans when the voltage applied to the input means of said differentialswitching means is between said first and second quiescent thresholdvoltages, means for connecting said output voltage control means theoutput means of said differential switching means and time-delay meansfor delaying the establishment of said second output voltage for a timegreater than the time required for the voltage applied to the inputmeans of said differential switching means to vary between said firstand second quiescent threshold voltages.
 12. A circuit for detecting thepresence or absence of an a-c voltage, in combination, a-c inputterminal means, output terminal means, amplifying means having first andsecond input means and output means, means for establishing first andsecond quiescent threshold voltages at said first and second input meansrespectively, first unidirectional conducting means for connecting saidfirst input means to said a-c input terminal means when the a-c voltageat said a-c input terminal means has a first polarity, secondunidirectional conducting means for connecting said second input meansto said a-c input terminal means when the voltage at said a-c inputterminal means has a second polarity, d-c voltage supply means, avoltage divider, means for connecting said input terminal means to saidd-c voltage supply means through said voltage divider, output voltagecontrol means, means for connecting said output voltage control means tosaid d-c voltage supply means and to the output means of said amplifyingmeans, time-delay means and means for connecting said time-delay meAnsto the output means of the said amplifying means and to said outputvoltage control means.